Fix signed int overflow in scanner

yylen is unsigned int, but len in zend_scan_escape_string() is int, which will
break for string literals >=2GB. yyleng is still limited to 4GB, but we can't
fix this without breaking the ABI.

Partially addresses phpGH-19542
Closes phpGH-19545

Author: iluuu1994

NEWS

@@ -5,6 +5,8 @@ PHP                                                                        NEWS
 - Core:
   . Fixed bug GH-18850 (Repeated inclusion of file with __halt_compiler()
     triggers "Constant already defined" warning). (ilutov)
+  . Partially fixed bug GH-19542 (Scanning of string literals >=2GB will fail
+    due to signed int overflow). (ilutov)
 
 - OpenSSL:
   . Fixed bug GH-19245 (Success error message on TLS stream accept failure).

Zend/zend_language_scanner.l

@@ -911,7 +911,7 @@ ZEND_API void zend_multibyte_yyinput_again(zend_encoding_filter old_input_filter
 		ZVAL_STRINGL(zendlval, yytext, yyleng); \
 	}
 
-static zend_result zend_scan_escape_string(zval *zendlval, char *str, int len, char quote_type)
+static zend_result zend_scan_escape_string(zval *zendlval, char *str, size_t len, char quote_type)
 {
 	char *s, *t;
 	char *end;